FA-69909 / Map projection transforms / Member archive

Slippy map tile index from coordinates: row lower clamp · case 04

Latitudes beyond the mercator limit produce negative row numbers.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Input [lon, lat, z] with lon in [-180, 180], any latitude, z >= 0. With n = 2**z tiles per axis, column = floor((lon+180)/360*n) and row = floor((1 - asinh(tan(lat))/pi)/2*n) (row 0 at the north edge); both are clamped to [0, n-1] so lon=180 falls in the last column and latitudes beyond the mercator limit fall in the edge rows. Return [z, column, row].

Why this case matters

Tile servers and caches key storage by these indices; an off-by-one or flipped axis fetches the wrong image.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
boundary #2	`[5, 16, -9]`	`[5, 16, 0]`	Failed

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